The present invention relates to a power steering mechanism for turning steerable wheels of a vehicle, and is particularly directed to a steering mechanism having a magnetoelastic torsion bar.
One known type of a power steering mechanism for turning steerable wheels of a vehicle includes a ball nut for transmitting force between an axially movable rack member and an electric motor. Upon actuation of the electric motor, the ball nut is driven to rotate relative to the rack member. The rotational force of the ball nut is transmitted to the rack member by balls to drive the rack member axially. Axial movement of the rack member effects turning movement of the steerable wheels.
In such a steering mechanism, the electric motor provides power steering assistance to a vehicle driver to accomplish the turning of the steerable wheels. This power assistance may be controlled in response to the torque applied to the vehicle""s steering wheel by the driver and other parameters such as vehicle speed. The steering mechanism includes an input shaft connected with the steering wheel and an output shaft operatively coupled with the rack member for mechanically moving the rack member axially. The input shaft and the output shaft are connected by a torsion bar which twists in response to steering torque applied to the steering wheel. When the torsion bar twists, relative rotation occurs between the input shaft and the output shaft.
Various angular displacement sensors are known for use in detecting applied steering torque between an input shaft and an output shaft. One known method for detecting applied steering torque utilizes a magnetoelastic ring attached to the torsion bar. Magnetic field sensors detect the application of torque to the torsion bar through the magnetoelastic ring. The ring, however, can be caused to rotate relative to the torsion bar under high torque conditions. Rotation of the ring relative to the torsion bar leaves residual stress in the steering mechanism and causes problems with signal degradation, accuracy, and zero shift. Thus, the elimination of such a ring is desirable.
Another known type of power steering mechanism uses an electric motor to rotate a pinion gear, which in turn drives the rack member axially, based on the applied steering torque and other parameters.
Other known steering mechanisms use a hydraulic motor and associated cylinder, instead of an electric motor, to provide power steering assist based on applied steering torque and other parameters.
The present invention is a power steering mechanism for turning steerable wheels of a vehicle. The steering mechanism comprises an input member rotatable about a first axis, an output member rotatable about a second axis coaxial with the first axis, and a torsion bar connecting the input member and the output member. The torsion bar is fixedly connected to the output member and twists upon relative rotation between the input member and the output member about the axes. The torsion bar is made of a magnetoelastic material and has a defined axially extending and circumferentially extending surface area which carries a magnetic field, which magnetic field varies upon twisting of the torsion bar. A magnetic field detector is located to sense variance in the magnetic field. The input member is interposed between the torsion bar and the magnetic field detector and is made of a non-magnetic material.